Exchange, IMAP and Netscape Mail

Situation: Randomly, users sending mail with or without attachments through Outlook to Netscape users, find that the text and attachment disappear. Nothing shows in the event log on the Exchange Server. After about 30 minutes, the text and attachment if any begin to populate the Netscape Mail message. This not happening everytime, but is random. No third party software seems to be involved (i.e. virus software). This is internal only, not to remote sites or Internet mail. Suggestions anyone?